The population of the UK is set to jump by the equivalent of a country the size of Ireland in the next decade, official figures have revealed.
Migration will fuel a projected rise of 4.4 million from 64.6 million last year to 69 million in 2024 and an estimated increase of just under 10 million over 25 years to reach 74.3 million in 2039.
It means the number of people living in Britain will swell by around the equivalent of the current populations of Ireland (4.6 million) and Sweden (9.6 million) over 10 years and a quarter of a century respectively.
Experts said that on current trends, the UK will overtake Germany to become the most populous country in Europe before the middle of this century.
More than two thirds (68%) of the long-term rise is due to future net migration and the indirect impact of children born to migrants, the Office for National Statistics (ONS) said.
It is assuming that annual net migration - the difference between the number of people entering and leaving the country - will run at an average of just under 200,000 a year over the 25 years.
The data also revealed a rapidly ageing society, with more than one in 12 people expected to be 80 or over by 2039.
Guy Goodwin, director of social analysis at the ONS, said: "The UK's population is set to grow and age.
"Growth will be at a faster rate than we have seen previously, largely due to the direct impact of international migration and the indirect impact of immigration.
"Despite this, the population will also be older as those born shortly after World War Two and during the 1960s 'baby boom' reach the oldest and pensionable ages respectively.
"The number of people of age 80 or over will more than double over the next 25 years."
Population projections, which are used to inform policies on pensions, migration, care and housing, are published every two years.
Compared to the last set of figures, which were based on 2012 estimates, experts have raised the projected growth over the next 10 years by just under a quarter of a million.
The average annual increase of 440,000 in the first decade means the population will rise by more than the number of people currently living in Dorset each year.
Britain's projected growth also far outstrips Europe, with an increase of 15% in the next 25 years compared to 3% in the EU.
Only Luxembourg, Belgium and Sweden are individually expected to see larger rises, although the projections for Europe were based on figures compiled prior to the recent migrant crisis.
If the current pattern continues, the UK will overtake France by 2030 and Germany, becoming the largest country in Europe, by 2047.
The figures prompted fresh scrutiny of ministers' attempts to reduce net migration to below 100,000. It stood at a record high of 330,000 in the year to March.
Lord Green, chairman of campaign group Migration Watch UK, said: "The prospect of nearly 10 million in 25 years underlines the huge impact on housing and public services, unless the Government succeeds in bringing net migration right down."
MPs Frank Field and Nicholas Soames, the co-chairmen of the cross-party Group on Balanced Migration, said the ONS projections "set out in stark terms the consequences of not reducing immigration to the UK".
They added: "It is not in the national interest to increase our population so rapidly. It is not of benefit to the existing residents of the UK. The country will become still more overcrowded and the housing crisis even worse."
Simon Ross, chief executive of charity Population Matters said it was "imperative" that factors contributing to the increase are addressed.
"We are all affected adversely by the rapid population growth of recent decades - from pressures on housing and public services to the environment and climate change," he said.